This invention relates to a method for improving fuel control in an internal combustion engine. More particularly, it relates to a method for improving the manner in which fuel is metered in an internal combustion engine fuel control system of the speed-density type.
There are two types of systems for controlling electrically the amount of fuel metered to an internal combustion engine. One of these is the mass air flow system, in which the volume or mass of air flowing into an engine is actually measured and the fuel is metered accordingly. The other system, speed-density, uses engine speed and the engine intake manifold absolute pressure to determine indirectly the amount of air entering an engine. In both types of electronic fuel control systems, the appropriate quantity of fuel is metered with a suitable fuel control apparatus. This apparatus typically has been a plurality of electromagnetic fuel injectors intermittently operated to deliver fuel into the intake manifold upstream of the usually provided intake valves.
In the speed-density fuel control system described in the aforementioned U.S. Pat. No. 4,086,884, the fuel control system employs a digital computer to calculate the amount of fuel required by the engine. The calculation is done repetitively to permit the fuel supply to be adjusted sufficiently often so that adequately precise control of fuel is achieved on a real-time basis. The computer preferably controls fuel in an interactive manner, that is, fuel supply, ignition timing and exhaust gas recirculation all are controlled simultaneously as interdependent output variables. The aforementioned U.S. Pat. No. 3,969,614 describes an interactive engine control system. In such a digital computer engine control system, an output variable, such as ignition timing, is taken into account in the determination of another output variable, such as the time and duration of injection in an intermittent-type fuel injection system. (If the injection is continuous, of course, determination of the usual points in the engine cycle at which injection is to be initiated is unnecessary.) The speed-density fuel injection system described in U.S. Pat. No. 4,086,884 requires that the volumetric efficiency of the engine be used, directly or indirectly, in the calculation of the quantity of fuel to be supplied to the engine. Unfortunately, the volumetric efficiency is a function of several parameters including engine speed and engine load. This means that these changing factors have had to be taken into account in the calculation of the quantity of fuel to be metered to the engine to satisfy the oxygen content of the intake mixture that actually enters the engine's combustion chambers. The desired fuel amount at any given time may, of course, be selected to provide a rich, a stoichiometric or a lean air/fuel mixture as may be required for engine operation in an open or closed-loop mode of engine operation.
A system using "speed-density" means that the system measures engine speed and intake charge density and a predetermined value for volumetric efficiency to calculate air flow. Based on dynamometer data for a particular 5 liter engine, the mathematical expression for this is: ##EQU1## where: MAP=Manifold Absolute Pressure from MAP sensor
RPM=Engine Speed--from CP sensor PA1 MCT=Manifold Charge Temperature (used for density correction) from MCT sensor. PA1 VEFF=Volumetric Efficiency for the engine from a look up table stored in the ECU. The value taken for VEFF is dependent on RPM and MAP.
To get the airmass from the above expression EGR mass must be subtracted. In an ideal engine having an efficiency, VEFF, of 1 and a more generalized proportionality constant, K, the equation reduces to EQU Total Mass Flow=(K)(MAP)(RPM)/MCT. (2)
This is a simple expression for the calculation of air mass flow using the speed-density approach That is, the speed is relating to engine RPM and the air density is related to pressure (MAP) and temperature (MCT). The proportionality constant K typically takes into account such factors as the volume of the engine, the number of cylinders filled per revolution and the units of the calculation.
The improved method of the present invention permits the volumetric efficiency of an engine having a speed-density electronic fuel control system to be determined much more accurately, under varying engine operating conditions, than is the case in the prior art systems. As a result, much more accurate fuel control is made possible and desired fuel economy and emission control benefits may be realized under certain circumstances.
The method of the invention improves fuel control in an internal combustion engine by providing for the computer calculation of an engine's current volumetric efficiency. The volumetric efficiency varies as a function of engine operating parameters, such as engine load, engine speed and other less significant variables. Specifically, the improved method of the invention comprises the steps of determining the ratio of the pressure in an engine's intake manifold to the absolute pressure of the products of combustion in a passage through which the products of combustion pass after leaving the engine's combustion chamber or chambers. This ratio of intake mixture and exhaust gas absolute pressures, or the inverse ratio, is combined mathematically with a second factor, which may be related to the engine speed, representative of forces acting upon the intake mixture as it flow toward the combustion chambers. The combined ratio and second factor are used to determine the volumetric efficiency of the engine with respect to the flow of gases into at least one combustion chamber thereof. This real-time volumetric efficiency then may be used to determine the amount of fuel metered to the engine.
The method of the invention is of value as compared to the prior art because of the simplicity and accuracy with which an engine's current volumetric efficiency can be determined. The ratio of the intake mixture and exhaust gas pressures is easily determined with the use of sensors typically found on engines having speed-density fuel control systems. Also, the engine speed is a variable that is readily available on a continuous basis in electronic engine control systems. The prior art speed-density systems, in contrast, have required the use of many time-consuming calculations, either digital or analog or both, based upon approximations of engine characteristics and design features. The system described in Moon et al. U.S. Pat. No. 4,086,884 mentioned above avoided this. The volumetric efficiency was treated as a function of temperature and pressure conditions in the intake manifold at the time the quantity of fuel to be delivered to the engine, i.e., the injector pulse width, was being calculated.
A very significant feature of the invention is that the real-time determination of volumetric efficiency takes into account the effects of changes in altitude on the characteristics of an engine's operation.